340 
PHYSIOLOGY. 
BOOK II. 
foramen of the ovule, through the intervention of the con- 
ducting tissue of the style. In ordinary cases this is easily 
effected, in consequence of the foramen being actually in 
contact with the placenta. Where it is otherwise, nature has 
provided some curious contrivances for bringing about the 
necessary contact. In Euphorbia Lalhyris the apex of the 
nucleus is protruded far beyond the foramen, so as to lie 
within a kind of hood-like expansion of the placenta : in all 
campylotropous ovules the foramen is bent downwards, by the 
unequal growth of the two sides, so as to come in contact 
with the conducting tissue ; and in Statice Armeria, Daphne 
Laureola, and some other plants, the surface of the con- 
ducting tissue actually elongates and stops up the mouth of 
the ovule, while fertilisation is taking effect. Another case 
occurs in Helianthemum. In plants of that genus the fora- 
men is at that end of the ovule which is most remote from 
the hilum ; and although the ovules themselves are elevated 
upon cords much longer than are usually met with, yet there 
is no obvious means provided for their coming in contact 
with any part through which the matter projected into the 
pollen tubes can be supposed to descend. It has, however, 
been ascertained by Adolphe Brongniart, that, at the time 
when the stigma is covered with pollen, and fertilisation has 
taken effect, there is a bundle of threads, originating in 
the base of the style, which hang down in the cavity of the 
ovary, and, floating there, are abundantly sufficient to convey 
the influence of the pollen to the points of the nuclei. So, 
again, in Asclepiadaceoe. In this tribe, from the peculiar 
conformation of the parts, and from the grains of pollen 
being all shut up in a sort of bag, out of which there seemed 
to be no escape, it was supposed that such plants must at 
least form an exception to the general rule. But before the 
month of November, 1828, the celebrated Prussian traveller 
and botanist, Ehrenberg, had discovered that the grains of 
pollen of Asclepiadaceae acquire a sort of tails, which are all 
directed to a suture of their sac on the side next the stigma, 
and which at the period of fertilisation are lengthened and 
emitted ; but he did not discover that these tails are only 
formed subsequently to the commencement of a new vital 
